A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonance frequencies, with greater amplitude than at others. Resonators can be used in, for example, crystal oscillators (also known as quartz oscillators), inductance-capacitive (LC) oscillators, resistance-capacitive (RC) oscillators, and Microelectromechanical systems (MEMS) oscillators, also referred to as micromechanical MEMS oscillators. A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. Crystal oscillators, such as quartz oscillators, are commonly used to generate frequencies to keep track of time (as in quartz clocks) or to generate a clock signal for digital integrated circuits. Usually, a different crystal is required for each desired frequency. Also, the crystal and the oscillator circuit compliments are typically distinct from one another, i.e., they are not integrated.
Resonators can also be used to select specific frequencies from a signal. Resonators can also be used in filters, such as in a quartz filter (also referred to as a crystal filter), or other piezoelectrics. Quartz resonators, for example, can directly convert their own mechanical motion into electrical signals. Quartz crystal filters have much higher quality factors than LCR filters (also referred to as a resonant circuit, tuned circuit, or RLC circuit) that has a resistor (R), an inductor (L), and a capacitor (C). These filters can be used in RF communication devices, for example, the quartz filter can be used in the intermediate frequency (IF) stages of a radio receiver. The quartz filter can be used for a fixed IF stage frequency because it has a very precise fixed frequency.
In general, electronic filters are electronic circuits which perform signal processing functions, specifically intended to remove unwanted signal components and/or enhance wanted ones. Electronic filters can be passive or active, analog or digital, discrete or continuous, linear or non-linear. The most common types of electronic filters are linear filters. One type of filter is a surface acoustic wave (SAW) filter, which is based on the transduction of acoustic waves. The transduction from electric energy to mechanical energy, in the form of SAWs, is accomplished by the use of piezoelectric materials. In particular, electrical signals are converted to a mechanical wave in a piezoelectric crystal. This wave is delayed as it propagates across the crystal, before being converted back to an electrical signal by further electrodes. The delayed outputs are recombined to produce a direct analog implementation of a finite impulse response filter. This hybrid filtering technique is also found in an analog sampled filter. Electronic devices, employing the SAW, typically utilize one or more inter-digital transducers (IDTs) to convert acoustic wave to electrical signal and vice versa utilizing the piezoelectric effect of certain materials, such as quartz. SAW filters are commonly used in radio-frequency (RF) applications, such as mobile telephones. These SAW filters may provide significant advantages in performance, cost, and size over other filter technologies such as quartz crystals (bulk wave), LC filters, and waveguide filters.
Tuning the center frequency of resonators and filters over a wide range can be difficult. For example, the MEMS resonator frequency is generally sensitive to electrostatic bias voltage, stress and any type of external force applied as control input. The higher the frequency the less sensitive the structure is to external forces, and the more difficult it is to pull its frequency.
Frequency pulling mechanisms are used to trim the frequency accuracy of oscillator and filters, to compensate for any type of drift including temperature, and to serve as a control mechanism for voltage controlled oscillators. These frequency pulling mechanisms may be, for example physical trimming, capacitive pulling, electrostatic pulling, stress or temperature control, or complex closed loop systems with multiple oscillators. For example, in oscillator applications, one can rely on the pulling effect of load capacitor. In such cases, the resonator mechanical frequency remains the same but the oscillator frequency is tuning over a very small range. For example, for quartz based oscillators, this capacitive load is used for temperature compensation and accurate trimming of the center frequency. In the case of MEMS oscillators, additional control can be exercised through the resonator DC Bias voltage (Vp) which creates an electrostatic pulling force. The pulling is directly proportional to the mechanical stiffness of the resonator. So, for a given control voltage range, the frequency pulling range will be reduced as the resonator increases in frequency. This control voltage can be used for trimming and temperature compensation.
Designs can be implemented to rely on stress dependence or temperature dependence of mechanical resonators to exercise a control over their center frequency. Physical trimming (etch, deposition) is used to bring the accuracy of resonator inside the specification range, especially for filters (quartz, thin Film Bulk Acoustic Resonator (FBAR), Bulk Acoustic Wave (BAW), MEMS, or the like.). More complex techniques for trimming and compensation of output oscillator at higher frequency have been introduced more recently. They rely on closed loop systems, like Fractional-N PLL or digital frequency discriminator where the main oscillator is locked to a reference oscillator, and their frequency ratio is control digitally. Other approaches may include electrostatic pulling in open-loop configuration, encapsulation of the device into micro-oven to keep temperature constant during fabrication, mechanical compensation of temperature drift, and Fractional-N PLL with reference drifting.
It should be noted that traditional electrostatic pulling is not effective in high-frequency MEMS oscillators. High-frequency MEMS resonators, such as MEMS resonators having approximately 1 MHz frequency or greater, for example, have a very high equivalent stiffness that causes them to have a very small electrostatic frequency pulling range. In MEMS oscillators, capacitive pulling, like used in quartz-based oscillators, may also not be effective to adjust the output frequency for both initial accuracy and temperature stability due to extremely small effective capacitance of the MEMS resonators.